The study is a double-blinded, placebo-controlled, randomized, multicenter, parallel-group dose finding study to investigate the efficacy and safety of four different doses of Cilansetron (1mg, 2mg, 8mg and 16 mg TID) to placebo in non-constipated patients with established irritable bowel syndrome. The primary efficacy parameter is the responder rate for IBS symptoms based on a weekly assessment of adequate relief of IBS symptoms (abdominal pain/discomfort, abnormal bowel habits.)